System and method of predicting the presence of an out-of-step condition in a power system

1. A method of predicting the presence of an out-of-step condition in a power system, the power system including a plurality of generators, the method including the steps of: (a) obtaining a differential value (δ COI k ) between a rotor angle (δ k ) of an individual one of the plurality of generators and an equivalent rotor angle (δ COI k ) of the centre of inertia of the remainder of the plurality of generators; (b) processing the differential value (δ COI k ) to determine whether the differential value (δ COI k ) is predicted to reach a predefined reference threshold (δ threshold ); and (c) predicting the presence of the out-of-step condition in the power system if the differential value (δ COI k ) is predicted to reach the predefined reference threshold (δ threshold ); wherein processing the differential value ({tilde over (δ)} COI k ) to determine whether the differential value ({tilde over (δ)} COI k ) is predicted to exceed a predefined reference threshold (δ threshold ) includes: (d) detecting the level of fluctuation of obtained differential values ({tilde over (δ)} COI k ); (e) fitting the obtained differential values ({tilde over (δ)} COI k ) to a curve if the level of fluctuation exceeds a fluctuation threshold; and (f) based at least in part on the fitting the obtained differential values ({tilde over (δ)} COI k ) to the curve, facilitating coordinated operation of the individual one generator of the plurality of generators with the plurality of generators.

2. A method according to claim 1 wherein in step (a) the equivalent rotor angle (δ COI k ) of the centre of inertia of the remainder of the plurality of generators is obtained by calculating a rotor angle (δ COI ) of the centre of inertia for the whole of the power system and deducing from this the said equivalent rotor angle (δ COI k ) by considering the rotor angle (δ k ) of the said individual generator.

3. A method according to claim 2 wherein the rotor angle (δ COI ) of the centre of inertia of the whole power system is calculated by a central control unit and thereafter transmitted to a local control unit of each generator, and each local control unit deduces from the calculated rotor angle (δ COI ) of the centre of inertia of the whole power system the corresponding said equivalent rotor angle (δ COI k ).

4. A method according to claim 3 wherein the central control unit receives time-stamped data from each generator to permit calculation of the rotor angle (δ COI ) of the centre of inertia of the whole power system and the calculated rotor angle (δ COI ) of the centre of inertia of the whole system is transmitted to the local control unit of each generator with a synchronous time stamp.

5. A method according to claim 1 wherein (d) detecting the level of fluctuation of obtained differential values includes (δ COI k ): establishing a coefficient of variation (C V ); and determining that the level of fluctuation has exceeded the fluctuation threshold when the coefficient of variation (C V ) exceeds a predetermined threshold coefficient value.

6. A method according to claim 5 wherein step (e) of fitting the obtained differential values (δ COI k ) to a curve includes fitting the sampled obtained differential values (δ COI k ) to a curve using a Prony method.

7. A method according to claim 6 wherein using the Prony method includes establishing an oscillation model of the power system having a rank which is determined by trial and error to minimise any error in the curve fitting.

8. The method according claim 1 wherein (b) processing the differential value (δ COI k ) to determine whether the differential value (δ COI k ) is predicted to exceed a predefined reference threshold additionally (δ threshold ) includes: (f) assessing the stability of obtained differential values (δ COI k ); and (g) predicting a future shape of the differential values curve if the stability of the obtained differential values (δ COI k ) is decreasing.

9. A method according to claim 8 wherein step (f) of assessing the stability of obtained differential values (δ COI k ) includes: decomposing each obtained differential value (δ COI k ) into a plurality of exponential polynomials having arbitrary amplitudes (A i ), phases (θ i ), frequencies (f t ) and decaying factors (α); and determining that the stability of the obtained differential values (δ COI k ) is decreasing if one or more of the decomposed exponential polynomials has a positive decay factor (α).

10. A method according to claim 8 wherein step (g) of predicting a future shape of the differential values curve includes: calculating a predicted trajectory of the differential values (δ COI k ); and calculating the rate of change of the real-time measured trajectory of the differential values (δ COI k ).

11. A method according to claim 10 wherein the predicted trajectory and rate of change calculations are carried out in respect of differential values (δ COI k ) obtained during a historical period.